This invention relates to a magnetron, and more particularly to a magnetron of the permanent magnet-externally disposed type wherein permanent magnets are disposed outside a main magnetron body.
Generally, a magnetron of the type wherein permanent magnets are disposed outside a main magnetron body, in other words outside an anode cylinder, or a so-called permanent-magnet-externally-disposed type magnetron, has the following construction for convenience of its manufacture.
Conventionally, the main magnetron body composed of the anode cylinder formed interiorly with resonance cavities, a cathode member disposed along the central axis of the anode cylinder, and a seal plate hermetically closing the opening of the anode cylinder is not secured to, for example, a frame-work especially by the use of a fixing means but is supported or sandwiched by forces applied thereto, respectively, through the permanent magnets which are disposed at upper and lower parts of the main magnetron body, respectively. These forces are applied through the frame work disposed outside the main magnetron body, whereby the frame work functions as means for holding indirectly the main magnetron body. For this frame work is substituted, for convenience of magnetron manufacture, a yoke magnetically coupling the pair of permanent magnets to each other, thereby to simplify the construction of the magnetron.
The reason for adopting the foregoing construction is to facilitate the assembling of the magnetron during its manufacturing process.
In recent years, however, as material of the permanent magnet of the conventional magnetron a rare earth magnet has come to be used in place of ferrite or the like. As this tendency becomes prominent, it has turned out difficult to adopt the magnetron having the above-mentioned construction for the reasons as later described.
Since the permanent magnet based on the use of a magnet comprising rare earth element-based compounds such as samarium cobalt, cerium cobalt, or the like is possessed of a great coercive force and large energy product, it is possible with a permanent magnet very small in volume to produce a desired magnetic field within the main magnetron body, and this provides an advantage that the magnetron itself can be substantially miniaturized. In spite of this advantage, however, the above-stated rare earth magnet has the drawback of being very low in mechanical strength as compared with an alnico or ferrite magnet usually used as the permanent magnet member of the conventional magnetron. Accordingly, in the case where, in the magnetron of the conventional construction having its main magnetron body sandwiched between a pair of permanent magnets by a frame work, the above-mentioned rare earth magnet is used as those permanent magnets, there is a risk that the permanent magnet is very likely to be damaged. Resultantly, the use of such rare earth magnet has been regarded as undesirable from the standpoint of magnetron manufacture.
In the foregoing description, the damages or breakages of the permanent magnet were explained as the drawbacks peculiar especially to the rare earth magnet. But where the force applied through the permanent magnet is of extremely large magnitude as a result of the errors made in the magnetron manufacturing process, there is similarly a risk that even a permanent magnet made of ferrite and a main magnetron body having high mechanical strength are easily damaged or broken.
Further, where, as in the conventional case, the magnetron is constructed into the type wherein a main magnetron body is sandwiched through a pair of permanent magnets, the heat generated from an anode cylinder during the operation of the magnetron is easily transferred to the permanent magnetron to produce not only a risk that the magnetic characteristics of the permanent magnet vary but also a disadvantage that difficulties are encountered in using means for preventing heat transfer. Especially in the case of the rare earth permanent magnet, its coercive force decreases with temperature elevation to a greater extent than in the case of the alnico magnet (though to a smaller extent than in the case of the ferrite magnet), thus to necessitate the use of any heat transfer preventing means. Accordingly, the use of the rare earth permanent magnet in the magnetron having the conventional construction is deemed unsuitable.